1. Field of the Invention
The invention relates to an article of manufacture which is a transmission signal produced by encoding a wideband digital signal of a specific sampling frequency F.sub.s, for example a digital audio signal, in an encoder, the transmission signal comprising consecutive frames, each frame comprising a plurality of information packets (IP), each information packet being N bits long, N being larger than 1, and a frame comprising at least a first frame portion including synchronization information. The invention also relates to an article of manufacture which is a record carrier in which such a signal is recorded.
2. Description of the Prior Art
A transmission signal as defined above can be used in a transmission system of the type known from the article, "The Critical Band Coder--Digital Encoding of Speech Signals Based on the Perceptual Requirement of the Auditory System" by M. E. Krasner, published in Proc. IEEE ICASSP 80, Vol. 1, pp 327-331, Apr. 9-11, 1980. This article relates to a transmission system in which the transmitter employs a subband coding system and the receiver employs a corresponding subband decoding system.
In this known coding system the speech signal band is divided into a plurality of frequency subbands whose bandwidths approximate the bandwidths of the critical bands of the human ear in the respective frequency ranges (see FIG. 2 of this article). This division is selected because, based on psycho-acoustic experiments, one can expect that quantization noise in such a subband will be masked to an optimum extent by the signals in that subband, if during quantization allowance is made for the noise-masking curve of the human ear. Threshold values for noise masking by single tones in the center of a critical band are shown in FIG. 3 of this article.
When applying frequency subband coding to a high-quality digital music signal, such as one according to the Compact Disc Standard which uses 16 bits per signal sample at a sample frequency of 1/T=44.1 kHz, with a suitably selected bandwidth and a suitable selected quantization for the respective subbbands, the quantized output signals of the coder can be represented by an average number of approximately 2.5 bits per signal sample. The quality of the replica of the music signal does not differ perceptibly from that of the original music signal in substantially all passages of substantially all kinds of music signals.
The subbands need not necessarily correspond to the bandwidths of the critical bands of the human ear. For example, the subbands may have equal bandwidths, provided that allowance is made for this in determining the masking threshold.
Other types of transmission signals are known with which the invention may be used, such as those in which blocks of samples are transform coded. Such systems are referred to in the article "Low bit-rate coding of high-quality audio signals. An introduction to the MASCAM system" by G. Thiele, G. Stoll and M. Link, published in EBU Technical Review, no. 230, pp. 71-94, August 1988. In such a system the transform coefficients correspond to sub-signals.
The sub-signal transmission systems described above have the disadvantage that, in some cases, perceptible differences occur between the replica and the original unencoded signal which was to be transmitted. These differences are perceived as a form of distortion in the replica generated by the receiver. Often they are the result of transmission signal or record carrier bandwidth requirements which limit the number of bits available for quantization of certain of the sub-signals.